In 2012, Darren Croft at the University of Exeter found evidence to support this hypothesis. His collaborator Ken Balcomb had been studying the resident killer whales of the Pacific Northwest since the 1970s; his astonishingly thorough census had captured the lives, deaths, and family ties of hundreds of these whales.
By ploughing through the data, student Emma Foster showed that if a male orca’s mother died before his thirtieth birthday, he was three times more likely to die the next year. If she passed away after he turned thirty, he was eight times more likely to subsequently snuff it. And if mum had gone through menopause, his odds of dying went up by fourteen times. The data were clear: mothers help their sons well into adulthood, and older mums are especially helpful.
“But that left a big unanswered question,” says Croft. “Old females are keeping their offspring alive, but how? What is it that they’re doing to confer the survival benefit?”
One reasonable guess involves salmon. Salmon makes up 97 percent of the diet of these particular orcas, and salmon are unpredictable. “They’re not distributed equally in space,” says Croft. “There are hotspots that differ with season, year, tide.” So just like human fishermen, the orcas need to know when and where to go to catch their fish. Do they stay at sea or swim inland? Do they go up their inlet or that one? The oldest females might be better at making these decisions, thanks to their accumulated experience.
To test this idea, the team turned to video footage of the southern residents, which Balcomb’s team had captured between 2001 to 2009. Postdoc Lauren Brent analysed over 750 hours of video to work out which whales were swimming together, and who was following whom. She also collected data from nearby fisheries to work out how big the salmon stocks were at different times.
She found that adult females are more likely to lead a group than adult males, and older post-menopasual females (who make up a fifth of the pod) were more likely to lead than younger ones. This bias was especially obvious in seasons when salmon stocks were low. And, as Foster found, there was a sex bias—males were more likely to follow their mother than females were.
These simple trends support the idea that the post-menopausal orcas are “repositories of ecological knowledge”. They lead the others to food, and their skills are especially important at times when food is scarce. And in doing so, they help their young to survive, which offsets the costs of forgoing any further reproduction. “That doesn’t tell us why they stop reproducing,” says Croft. “You could share information while still being reproductive. Why did they stop? That’s the next question.”
The same principles apply to human menopause, too. Some scientists have suggested that human menopause is merely a side effect of our longer lifespans, brought about by medicine and sanitation. But that can’t be right. Among many hunter-gatherers, like the Ache of Paraguay or the Hadza of Tanzania, around half of women survive to 45, and continue living into their late 60s. Like killer whales, they live long after the stop reproducing. And like killer whales, the longer they live, the more they know. In 2001, anthropologist Jared Diamond wrote:
“Old people are the repositories of knowledge in preliterate societies. In my field studies of New Guinea birds, I start work in a new area by gathering the oldest hunters and quizzing them… When the hunters are stumped by my asking about some especially rare bird, they answer: “We don’t know, let’s ask the old man (or woman).” We go into another hut, where we find a blind and toothless old person who can describe a rare bird last seen 50 years ago. Some of that stored information is essential to the survival of the whole village, whose members include most living relatives of the old person. The information encompasses wisdom about how to survive dangers — such as droughts, crop failures, cyclones and raids — that occur at long intervals but that could kill the whole tribe if it did not know how to react.”
Why, then, don’t elephants go through menopause? They are also long-lived animals that stay in family groups, and the old females—the matriarchs—are vital. They are better at recognising friendly faces and they know the best anti-lion moves. They provide their herds with the same benefits that orcas like Granny bestow upon their pods.
But resident killer whales differ from elephants in one critical respect: their sons and daughters stay in the groups where they were born. This means that as a female grows older, her pod becomes increasingly full of her own children and grandchildren. Over time, she becomes increasingly related to her neighbours, and she shares more and more of her genes with her neighbours. This creates a powerful impetus to shift her efforts away from having more children, and towards helping her existing descendants.
That impetus doesn’t exist in elephants because their sons eventually leave their birth group to find new ones. Females become less related to their group-mates over time or, at least, no more related. A matriarch’s best bet, then, is to carry on reproducing until she dies.
And humans? Many anthropologists believed that we started off with female-biased dispersal—that is, daughters would leave to join new groups. “When she joins, she has zero relatedness to the rest of the group,” explains Croft. “But as she ages, she has offspring and her local relatedness increases.” Then again, other animals like hamadryas baboons and the Seychelles warbler also have female-biased dispersal and don’t go through menopause. “So, it’s not just about the dispersal patterns but also the role that old females can play in the group,” says Croft.
In killer whales, the old females might also be better at catching salmon, which they then share with their kin. Perhaps they understand the hierarchies and structures of other groups, and mediate fights between their sons and rivals. These ideas are harder to test. “We have so little information on them,” says Croft. “We see them at the surface and we know so little about their lives.”
Reference: Brent, Franks, Foster, Balcomb, Cant & Croft. 2015. Ecological Knowledge, Leadership, and the Evolution of Menopause in Killer Whales. Current Biology http://dx.doi.org/10.1016/j.cub.2015.01.037